CBSE Class 10 Science HOTs Carbon And its Compounds Set 03

Question. Write the names and molecular formula of two organic compounds having functional group suffixed as ‘–oic acid’. With the help of a balanced chemical equation and explain what happens when any one of them reacts with sodium hydroxide.
Answer: HCOOH (Methanoic acid), its molecular formula is \( CH_2O_2 \).
\( CH_3COOH \) (Ethanoic acid), its molecular formula is \( C_2H_4O_2 \).
When acid reacts with sodium hydroxide, its sodium salt and water is formed.
\( CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O \)

Question. What is a homologous series? Which two of the following organic compounds belong to the same homologous?
\( C_2H_6, C_2H_6O, C_2H_6O_2, CH_4O \).
Answer: Homologous series is a series of organic compounds which have same functional group and similar chemical properties. Each member of this series is differ by —\( CH_2 \)— in its molecular formula and 14 u in its molecular mass.
\( C_2H_6O (C_2H_5OH) \) and \( CH_4O (CH_3OH) \) belong to same homologous series.

Question. What is the IUPAC name of
(i) \( CH_3-CH_2-CH=CH_2 \)
(ii) \( CH_3CHO \)?
Answer: (i) But-1-ene (ii) Ethanal

Question. Heating an alcohol with concentrated sulphuric acid results in the dehydration of the alcohol to give the alkene as shown by the reaction of ethanol to give ethene.
\( CH_3CH_2OH \xrightarrow{\text{Hot conc. sulphuric acid}} CH_2=CH_2 \)
Pramila heated 2-butanol with concentrated sulphuric acid. Write the structural formulae of all the possible products of the reaction.
Answer: \( CH_3CH=CHCH_3 \), But-2-ene
\( CH_3CH_2CH=CH_2 \), But-1-ene

Question. Organic compounds belonging to different homologous series can be isomers. For example, propanal and propanone are isomers. Can an alkane and an alcohol be isomers? Why or why not?
Answer: No, they cannot be isomers because isomers must have same molecular formula.
– Alkanes have only carbon and hydrogen atoms, while alcohols have oxygen atoms too.

Question. The number of carbon compounds is more than those formed by all other elements put together. Justify the statement by giving two reasons.
Answer: • Due to self linking ability of carbon/catenation.
• Since carbon has a valency of four it can form bonds with four other atoms of carbon or atoms of some other mono-valent element.
• Due to small size of carbon it forms very strong and (or) stable bonds with other elements.

Question. Alkanes are saturated compounds of carbon and hydrogen that can be represented by the general formula \( C_nH_{2n+2} \) where ‘n’ is the number of carbon atoms. An example of such a compound is ethane \( C_2H_6 \).
Maya has a compound of carbon and hydrogen whose formula is \( C_3H_4 \).
(a) What is true about the type of flame this compound will give on combustion?
(b) Draw all the possible straight chain structures of this compound.
Answer: (a) The compound being unsaturated will burn with a sooty or smoky flame.
(b) \( CH_2=C=CH_2 \) (Propa-1,2-diene) and \( H-C \equiv C-CH_3 \) (Propyne).

Question. A carbon compound of molecular formula \( C_5H_{10}O \) contains a ketone functional group. Draw the structures of three isomers of this compound having a ketone group.
Answer:
\( CH_3-CH_2-CH_2-CO-CH_3 \) (Pentan-2-one)
\( CH_3-CH_2-CO-CH_2-CH_3 \) (Pentan-3-one)
\( CH_3-CH(CH_3)-CO-CH_3 \) (3-Methyl butan-2-one)

Question. Ethanol (\( C_2H_5OH \)) is heated with alkaline potassium permanganate to give a compound X.
\( C_2H_5OH \xrightarrow{\text{alkaline } KMnO_4 + \text{heat}} X \)
(a) How many carbon atoms will compound X contain?
(b) Compound X is now reacted with ethanol in the presence of an acid catalyst to give a compound Y.
\( X + C_2H_5OH \xrightarrow{acid} Y \)
(i) Name the type of compound formed in the above reaction with respect to the functional group it contains.
(ii) State one characteristic property of compounds of the type of compound Y.
(iii) State one use of compounds of this type.
Answer: (a) Two. \( CH_3COOH \) ('X').
(b) (i) ester. \( CH_3COOC_2H_5 \) ('Y').
(ii) Esters are pleasant fruity smelling compounds.
(iii) Perfumes or flavouring agents in ice creams, cold drinks etc.

Question. Compounds with identical molecular formula but different structures are called structural isomers.
(a) In the case of saturated hydrocarbons, what is the MINIMUM number of carbon atoms needed in a molecule for it to have a structural isomer?
(b) Draw the structural isomers of the saturated hydrocarbon having the minimum number of carbon atoms mentioned in (a).
Answer: (a) Four. \( C_4H_{10} \) will show isomerism.
(b) \( CH_3-CH_2-CH_2-CH_3 \) (Butane) and \( CH_3-CH(CH_3)-CH_3 \) (2-methyl propane).

Question. An open-chain hydrocarbon X having the general formula of \( C_nH_{2n-2} \) is hydrogenated in the presence of a catalyst.
(a) State the number of moles of hydrogen required to completely saturate 1 mole of compound X.
(b) The hydrocarbon X contains carbon-carbon single bonds. Apart from the single bonds, state the number and the type of other carbon-carbon bonds that could possibly be present in the compound X.
Answer: (a) 2 moles. \( R-C \equiv CH + 2H_2 \rightarrow R-CH_2-CH_3 \)
(b) – two C—C double bonds or – one C—C triple bond.

Question. Shown below are the structural formulae of four carbon compounds.
P: \( CH_3-CO-CH_3 \)
Q: \( CH_3-CH(OH)-CH_3 \)
R: \( CH_3-CH_2-COOH \)
S: \( CH_3-OH \)
(a) Two of these compounds are more likely to have similar chemical properties. Identify these two compounds. Give a reason for your answer.
(b) Identify which of these compounds are likely to have the same boiling point. Justify your answer.
Answer: (a) Q and S because both belong to same family of alcohols as they have the same functional group.
(b) None of them because different compounds differ in physical properties like boiling point. They are all different chemical substances.

Question. Manasi wrote the names of four compounds as the first members of their respective homologous series.
– Methanol – Methanal
– Methanone – Methanoic acid
(a) Which name has she written incorrectly? Justify your answer.
(b) What name should she have written instead?
Answer: (a) Methanone is not possible because the smallest ketone has three carbon atoms.
(b) Propanone.

Question. (a) How many isomers are possible for the compound with the molecular formula \( C_4H_8 \)? Draw the electron dot structure of branched chain isomer.
(b) How will you prove that \( C_4H_8 \) and \( C_5H_{10} \) are homologues?
Answer: (a) Four. The branched isomer is 2-methylpropene \( CH_2=C(CH_3)_2 \).
(b) \( C_4H_8 \) and \( C_5H_{10} \) are homologues as they:
• differ in —\( CH_2 \)— group
• differ in 14u molecular mass
• have same functional group
• have same general formula

Question. What is an oxidising agent? What happens when an oxidising agent is added to propanol? Explain with the help of a chemical equation.
Answer: Those substances which add oxygen are called oxidising agent.
\( CH_3CH_2CH_2OH + 2[O] \xrightarrow{\text{Alkaline } KMnO_4/H_2SO_4} CH_3CH_2COOH + H_2O \)
Propanol will get oxidised to propanoic acid by acidified \( KMnO_4 \).

Question. An aldehyde as well as ketone can be represented by the same molecular formula, say \( C_3H_6O \). Write their structures and name them. State the relation between the two in the language of science.
Answer: \( CH_3-CH_2-CHO \) (Propanal) and \( CH_3-CO-CH_3 \) (Propanone). They are functional isomers.

Question. \( C_3H_6, C_4H_8 \) and \( C_5H_{10} \) belong to the same homologous series.
(a) Define homologous series.
(b) Why the melting and boiling points of \( C_5H_{10} \) is higher than \( C_4H_8 \)?
(c) Arrange these hydrocarbons in order of increasing boiling points.
Answer: (a) The series of organic compounds which have similar chemical properties and the same functional group is called homologous series.
(b) It is because \( C_5H_{10} \) has higher molecular weight, more Van der Waal’s force of attraction and higher boiling points and melting points.
(c) \( C_3H_6 < C_4H_8 < C_5H_{10} \)

Question. What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties.
Answer: Those compounds which are formed by sharing of electrons are called covalent compounds. They differ from ionic compounds because they do not have ions.
Properties:
(i) They have low melting and boiling points.
(ii) They do not conduct electricity in molten state or in aqueous solution.
(iii) They are mostly insoluble in water but soluble in organic solvents.

Question. Complete the following chemical equations:
(a) \( CH_3COOC_2H_5 + NaOH \rightarrow \)
(b) \( CH_3COOH + NaOH \rightarrow \)
(c) \( C_2H_5OH + CH_3COOH \xrightarrow{\text{Conc. } H_2SO_4} \)
Answer:
(a) \( CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH \)
(b) \( CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O \)
(c) \( C_2H_5OH + CH_3COOH \xrightarrow{\text{Conc. } H_2SO_4} CH_3COOC_2H_5 + H_2O \)

Question. An ester has the molecular formula \( C_4H_8O_2 \). Write its structural formula. What happens when this ester is heated in the presence of sodium hydroxide solution? Write the balanced chemical equation for the reaction and name the products. What is a saponification reaction?
Answer: There are three possible structural formulae of esters with molecular formula \( C_4H_8O_2 \).
\( CH_3CH_2COOCH_3 \), \( HCOOCH_2CH_2CH_3 \), \( CH_3COOC_2H_5 \) are isomers.
\( CH_3CH_2COOCH_3 + NaOH \rightarrow CH_3CH_2COONa + CH_3OH \)
Methyl propanoate + Sodium hydroxide \( \rightarrow \) Sodium propanoate + Methanol
or
\( HCOOCH_2CH_2CH_3 + NaOH \rightarrow HCOONa + CH_3CH_2CH_2OH \)
Propyl methanoate + Sodium hydroxide \( \rightarrow \) Sodium methanoate + 1-Propanol
or
\( CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH \)
Ethyl ethanoate + Sodium hydroxide \( \rightarrow \) Sodium ethanoate + Ethanol
Saponification is the process in which an ester is treated with sodium hydroxide to form sodium salt of acid and alcohol is formed.

Ethanol is a renewable biofuel because it is made from biomass. Ethanol is a clear, colourless alcohol made from a variety of biomass materials. Ethanol producers mostly use food grains and crops with high starch and sugar content such as corn, sorghum, barley, sugar cane, and sugar beets. The most common ethanol production processes today use yeast to ferment the starch and sugars in corn, sugar cane, and sugar beets.

Question. What is the chemical formula for ethanol?
Answer: \( CH_3CH_2OH \)

Question. What other compound is obtained as a by-product when ethanol is obtained from a sugar?
Answer: carbon dioxide (\( CO_2 \))

Question. What would be the products formed when ethanol undergoes complete combustion? Support your answer with a balanced chemical equation.
Answer: carbon dioxide and water
\( CH_3CH_2OH + 3O_2 \rightarrow 2CO_2 + 3H_2O \)

Long Answer Type Questions

Question. Give reasons for the following:
(a) Element carbon forms compounds mainly by covalent bonding.
(b) Diamond has a high melting point.
(c) Graphite is a good conductor of electricity.
(d) Acetylene burns with a sooty flame.
(e) Kerosene does not decolourise bromine water while cooking oils do.
Answer: (a) It is because carbon has four valence electrons, it cannot gain or lose four electrons because high energy is needed. It can only share four electrons.
(b) It is due to strong covalent bonds and compact structure of diamond.
(c) It is due to presence of free electrons in graphite because each carbon is linked to three more carbon atoms.
(d) It is due to high percentage of carbon, it burns with sooty or smoky flame.
(e) Kerosene oil is mixture of saturated hydrocarbons therefore does not decolourise bromine water.

Question. (a) Give a chemical test to distinguish between saturated and unsaturated hydrocarbon.
(b) Name the products formed when ethane burns in air. Write the balanced chemical equation for the reaction showing the types of energies liberated.
(c) Why is reaction between methane and chlorine in the presence of sunlight considered a substitution reaction?
Answer: (a) Add bromine water. Unsaturated hydrocarbon will decolourise bromine water whereas saturated hydrocarbon will not react.
(b) Carbon dioxide and water are formed.
\( 2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O + Heat + Light \)
(c) \( CH_4(g) + Cl_2(g) \xrightarrow{Sunlight} CH_3Cl(g) + HCl(g) \)
It is because hydrogen atom of methane gets substituted by chlorine atom to form chloromethane, therefore, it is called substitution reaction.

Question. List in tabular form three physical and two chemical properties on the basis of which ethanol and ethanoic acid can be differentiated.
Answer:
Physical properties:
- Ethanol has a specific smell; Ethanoic acid has a vinegar-like smell.
- Ethanol has a burning taste; Ethanoic acid is sour in taste.
- Ethanol does not freeze in winters; Ethanoic acid freezes in winters.

Chemical properties:
- Ethanol does not react with \( NaHCO_3 \); Ethanoic acid gives \( CO_2 \) with \( NaHCO_3 \).
- Ethanol burns with a blue flame; Ethanoic acid does not burn with a blue flame.
- Ethanol does not affect blue litmus; Ethanoic acid turns blue litmus red.

Question. Shristi heated Ethanol with a compound A in presence of a few drops of concentrated sulphuric acid and observed a sweet-smelling compound B is formed. When B is treated with sodium hydroxide it gives back Ethanol and a compound C.
(a) Identify A and C
(b) Give one use each of compounds A and B.
(c) Write the chemical reactions involved and name the reactions.
Answer: (a) A – Ethanoic acid/ Or any other carboxylic acid, C- Sodium salt of ethanoic acid/ any other carboxylic acid/ sodium ethanoate
(b) Use of A- dil solution used as vinegar in cooking/ preservative in pickles. Use of B – making perfumes, flavoring agent.
(c) \( CH_3COOH + C_2H_5OH \xrightarrow{Conc. H_2SO_4} CH_3COOC_2H_5 + H_2O \) (Esterification reaction)
\( CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH \) (Saponification reaction)

Question. (a) What is the role of concentrated sulphuric acid when it is heated with ethanol at 443 K? Give the reaction involved.
(b) Reshu by mistake forgot to label the two test tubes containing ethanol and ethanoic acid. Suggest an experiment to identify the substances correctly. Illustrate the reactions with the help of chemical equations.
Answer: (a) Sulphuric acid acts as dehydrating agent.
\( C_2H_5OH \xrightarrow{Conc. H_2SO_4, 443 K} C_2H_4 + H_2O \)
(b) By reaction with sodium carbonate/ bicarbonate with the samples, ethanol will not react whereas ethanoic acid gives brisk effervescence.
\( 2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2 \)
or
\( CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2 \)

Question. The formulae of four organic compounds are given below:
A: \( C_2H_4 \)
B: \( CH_3COOH \)
C: \( C_2H_5OH \)
D: \( C_2H_6 \)
(a) Which one of these compounds A, B, C or D is a saturated hydrocarbon?
(b) Identify the organic acid and give its structural formula.
(c) Which of the above compounds when heated at 443K in the presence of concentrated \( H_2SO_4 \) forms ethene as the major product? What is the role played by concentrated \( H_2SO_4 \) in this reaction? Also write the chemical equation involved.
(d) Give a chemical equation when B and C react with each other in presence of concentrated \( H_2SO_4 \). Name the major product formed and mention one of its important use.
Answer: (a) D is a saturated hydrocarbon.
(b) B is an organic acid. Structural formula: \( CH_3-COOH \)
(c) C (Ethanol). It acts as a dehydrating agent and removes a water molecule from ethanol.
\( C_2H_5OH \xrightarrow{hot Conc. H_2SO_4} C_2H_4 + H_2O \)
(d) \( CH_3COOH + C_2H_5OH \xrightarrow{Conc. H_2SO_4} CH_3COOC_2H_5 + H_2O \)
Major product is ester and it is used in making perfumes/flavouring agents.

Question. (a) What is homologous series of carbon compounds? Write general formula of alkynes. Name and draw the electron dot structure of first homologue of this series.
(b) State the meaning of the functional group in an organic compound. Write the formula of functional group present in alcohols and carboxylic acids.
Answer: (a) The series of organic compounds having same functional group, similar chemical properties, gradation in physical properties are called homologous series.
General formula of alkynes \( C_nH_{2n–2} \)
First homologue: Ethyne (\( C_2H_2 \)). Electron dot structure: \( H : C \vdots \vdots C : H \)
(b) Functional group is an atom or group of atoms or reactive part of compound which determines chemical properties of organic compounds e.g., —OH is present in alcohols, —COOH is functional group in carboxylic acids.

Question. (a) It is observed that covalent compounds are bad conductor of electricity. Give reason.
(b) Carbon can neither form \( C^{4+} \) cation and nor \( C^{4-} \) anion. Explain.
(c) Draw electron dot structure of ethanol.
(d) Identify the heteroatom (s) in the following compounds.
(i) \( CH_3CH_2-CO-CH_3 \)
(ii) \( CH_3CH_2Cl \).
Answer: (a) It is because covalent compounds do not form ions therefore do not conduct electricity.
(b) Carbon cannot lose 4 electrons to form \( C^{4+} \) cation because very large amount of energy is required. It cannot gain 4 electrons to form \( C^{4-} \) anion because 6 protons can not hold 10 electrons.
(c) Electron dot structure of ethanol \( CH_3CH_2OH \):
\( H(3) - C - C - O - H \)
(d) (i) oxygen, (ii) chlorine

PRACTICE QUESTIONS

Question. Which amongst the following contains triple bond?
(a) \( C_2H_4 \)
(b) \( C_2H_6 \)
(c) \( C_2H_2 \)
(d) \( C_4H_8 \)
Answer: (c) \( C_2H_2 \)

Question. Which of the following are members of same homologous series?
(a) \( HCOOCH_3 \) and \( CH_3COOCH_3 \)
(b) \( HCOOH \) and \( HCOOCH_3 \)
(c) \( CH_3COCH_3 \) and \( CH_3CH_2CHO \)
(d) \( C_2H_5OH \) and \( CH_3OCH_3 \)
Answer: (a) \( HCOOCH_3 \) and \( CH_3COOCH_3 \)

Question. Which of the following represents cyclohexane?
(a) \( C_6H_{14} \)
(b) \( C_6H_{12} \)
(c) \( C_6H_{10} \)
(d) \( C_6H_6 \)
Answer: (b) \( C_6H_{12} \)

Question. A student studies \( CH_3-COOH \) is saturated compound. The reason is
(a) there is single bond between carbon atoms.
(b) there is double bond between carbon and oxygen atoms.
(c) there is single bond between carbon and hydrogen atoms.
(d) there is single bond between carbon and hydroxide diatoms.
Answer: (a) there is single bond between carbon atoms.

Question. What is allotropy? Give example.
Answer: Allotropy is the property of some chemical elements to exist in two or more different forms, in the same physical state. Example: Diamond and Graphite are allotropes of carbon.

Question. Name the following compounds:
(a) \( CH_3-CH_2-OH \), (b) \( CH_3-CHO \)
Answer: (a) Ethanol, (b) Ethanal

Question. (a) Write the general formula of hydrocarbon alkene. Write the name of simplest alkene.
(b) Write the number of covalent bonds in the molecule of butane, \( C_4H_{10} \).
Answer: (a) General formula: \( C_nH_{2n} \). Simplest alkene: Ethene (\( C_2H_4 \)).
(b) There are 13 covalent bonds in butane.

Question. (a) Name the first member of ketone.
(b) Write the molecular formula of benzene and state the number of double bonds in its structure.
Answer: (a) Propanone.
(b) Molecular formula: \( C_6H_6 \). It has 3 double bonds.

Question. Write the molecular formula of the first two members of the homologous series having functional group —COOH.
Answer: \( HCOOH \) (Methanoic acid) and \( CH_3COOH \) (Ethanoic acid).

Question. Write the name and formula of the \( 3^{rd} \) member of homologous series having general formula \( C_nH_{2n} \).
Answer: Butene, \( C_4H_8 \).